1. Field of the Invention
This invention relates to separation processes and in particular the use of a mixed matrix membrane to achieve such separation of a monosaccharide from a polysaccharide.
2. Background Information
In recent years reverse osmosis has attracted a great deal of interest for utilization in fields involving purification of liquids. This is of especial importance when utilizing this system in the purification of water and especially saline water. Likewise, the process is also used to remove impurities from liquids such as water or, in the field of dialysis, blood. When utilizing reverse osmosis in the purification of a saline water, a pressure in excess of the osmotic pressure of the saline water feed solution is applied to the solution which is prepared from purified water by the semipermeable membrane. Pure water thereby diffuses through the membrane while the sodium chloride molecules or other impurities which may be present in the water are retained by the membrane.
Various semipermeable membranes are now being used in commercial processes for performing separations by the reverse osmosis treatment of aqueous solutions either for the portion of relatively pure water or for concentration of a liquid solution being treated or both. Such semipermeable membranes which are being used commercially include the early Loeb-type membranes made of cellulose diacetate by processes such as described in U.S. Pat. Nos. 3,133,132 to Loeb et al. and 3,133,137 to Loeb et al. The Loeb-type membranes comprise the asymmetric type which are characterized by a very thin, dense surface layer or skin that is supported upon an integrally attached, much thicker supporting layer. Other types of semipermeable membranes which are also in use include membranes having been fabricated from polyamides, polyimides, polyphenyl esters, polysulfone, polybenzimidazole, polyarylene oxides, polyvinylmethyl ether and other polymeric organic materials.
It is taught in U.S. Pat. No. 4,243,701 to Riley et al. that certain membranes may also be utilized for the separation of various gases. The separation of a gas mixture utilizing a membrane is effected by passing a feed stream of the gas across the surface of the membrane. Inasmuch as the feed stream is at an elevated pressure relative to the effluent stream, a more permeable component of the mixture will pass through the membrane at a more rapid rate than will a less permeable component. Therefore, the permeate stream which passes through the membrane is enriched in the more permeable component while, conversely, the residue stream is enriched in the less permeable component of the feed.
The use of adsorbents or molecular sieves in separating components from fluid mixtures is also long known. In the adsorption type separation process the adsorbent exhibits selectivity of one mixture component over another or, with a molecular sieve, one component is more retained than another. The adsorbent may be employed in the form of a dense compact fixed bed which is alternatively contacted with the feed mixture and desorbent materials. In the simplest case, the adsorbent is employed in the form of a single static bed in which case the process is only semi-continuous. In another embodiment, a set of two or more static beds may be employed in fixed bed contacting with appropriate valving so that the feed mixing is passed through one or more adsorbent beds, while the desorbent materials can be passed through one or more of the other beds in the set. The flow of feed mixture and desorbent materials may be either up or down through the adsorbent.
The most commercially successful embodiment of the adsorptive type separation process is the countercurrent moving-bed of simulated moving-bed countercurrent flow system. In that system the adsorption and desorption operations are continuously taking place which allows both continuous production of an extract and raffinate stream and the continual use of feed and desorbent streams. The operating principles and sequence of such a flow system are described in U.S. Pat. No. 2,985,589 to Broughton et al.
It is known in the separation art that certain crystalline aluminosilicates referred to as zeolites can be used in the separation of a component from an aqueous solution of a mixture of different components. For example, adsorbents comprising aluminosilicate are used in the method described in U.S. Pat. No. 4,014,711 to Odawara to separate fructose from a mixture of sugars in aqueous solution including fructose and glucose.
It is also known that crystalline aluminosilicates or zeolites are used in adsorption processing in the form of agglomerates having high physical strength and attrition resistance. Methods for forming the crystalline powders into such agglomerates include the addition of an inorganic binder, generally a clay comprising silicon dioxide and aluminum oxide to the high purity zeolite powder in wet mixture. The blended clay zeolite mixture is extruded into cylindrical type pellets or formed into beads which are subsequently calcined in order to convert the clay to an amorphous binder of considerable mechanical strength. As binders, clays of the kaolin type are generally used. It is also known that water permeable organic polymers such as cellulose acetate are superior binders.
A new composition of matter referred to as "silicalite", has recently been developed and patented (see U.S. Pat. No. 4,061,724 to Grose et al.). Silicalite is a hydrophobic crystalline silica molecular sieve. Due to its aluminum-free structure, silicalite does not show ion-exchange behavior, and is hydrophobic and organophilic. Silicalite thus comprises a molecular sieve, but not a zeolite. Silicalite is uniquely suitable for many separations processes for the presumed reason that its pores are of a size and shape that enable the silicalite to function as a molecular sieve, i.e., accept the extract molecules into its channels or internal structure, while rejecting the raffinate molecules. A detailed discussion of silicalite may be found in the article "Silicalite, A New Hydrophobic Crystalline Silica Molecular Sieve"; Nature, Vol. 271, 9 Feb. 1978, incorporated herein by reference.
There are numerous reference which disclose the incorporation of various materials with separation membranes. U.S. Pat. Nos. 3,457,170 to Havens; 3,878,104 to Guerrero; 3,993,566 to Goldberg et al.; 4,032,454 to Hoover et al.; and 4,341,605 to Solenberger et al. teach the use of structural supports or reinforcement fibers or fabrics to aid the membrane in resisting the high pressures used in the reverse osmosis process. U.S. Pat. No. 3,556,305 to Shorr shows a "sandwich" type reverse osmosis membrane comprising a porous substrate covered by a barrier layer, in turn covered by a polymer or film bonded to the barrier layer by an adhesive polymeric layer. U.S. Pat. No. 3,862,030 to Goldberg shows a polymeric matrix having an inorganic filler such as silica dispersed throughout which imparts a network of micro-voids or pores of about 0.01 to about 100 microns, capable of filtering microscopic or ultrafine particles of sub-micron size. U.S. Pat. No. 4,116,889 to Chlanda et al. discloses a bipolar membrane comprising a layer of ion exchange resin and a layer of particles of an ion exchange resin in a matrix polymer, useful for electrodialytic water splitting. U.S. Pat. No. 4,302,334 to Jakabhazy et al. discloses a membrane "alloy" comprising a hydrophobic fluorocarbon polymer blended with polyvinyl alcohol polymer which imparts hydrophilic properties to the membrane. U.S. Pat. No. 4,340,428 to Boddeker et al. discloses a membrane comprising a swollen organophilic bentonite in a cellulose acetate polymer for use in water desalting.
Mixed matrix membranes such as molecular sieves incorporated with polymeric membranes are also broadly disclosed in the art. In the article "The Diffusion Time Lag in Polymer Membranes Containing Adsorptive Fillers" by D. R. Paul and D. R. Kemp, J. Polymer Sci.; Symposium No. 41, 79-93 (1973), the specific mixed membrane used was a Type 5A (Linde) zeolite incorporated with a silicone rubber matrix. The Paul et al. article illustrates that the zeolite "filler" causes changes in the time lag for reaching steady state permeation of the membrane by various gases due to the differences in adsorption of the gases by the zeolite. It is taught in this article that once the zeolite becomes saturated by the permeating gas a steady state selectivity (ratio of permeation rates) through the membrane is reached substantially the same as if the zeolite was not present. The Paul et al. article teaches making the mixed matrix membrane by dispersing the molecular sieves into the fluid silicone prepolymer prior to casting.
U.S. Pat. No. 2,924,630 to Fleck et al. broadly discloses the use of a zeolitic barrier material which may be mechanically pressed into or deposited in the pores of a fluid-permeable material to provide a means for separating molecules of different sizes.
U.S. Pat. No. 3,817,232 to Nakajima et al. discloses the use of either a zeolite or a nitrogen impermeable membrane for the separation of oxygen from nitrogen.
U.S. Pat. No. 4,305,782 to Ostreicher et al. discloses the use as a filter media of activated carbon or molecular sieves in a self-bonding matrix of cellulose fiber.
We have discovered a novel and highly advantageous method of using a particular mixed matrix membrane for the separation of a monosaccharide from a polysaccharide not disclosed by any of the known art either alone or in combination.